1. Field
The present disclosure generally relates to the fabrication of an optical facet on a substrate. More specifically, the present disclosure relates to fabrication of an optical facet on a substrate by defining a trench on the back side of the substrate.
2. Related Art
Optical signaling based on silicon photonics has the potential to alleviate off-chip bandwidth bottlenecks, as well as to provide low latency chip-to-chip communication. Optical interconnects with these capabilities can facilitate new system architectures that include multiple chips, with multi-threaded cores. These optical interconnects can provide: high-bandwidth, low-latency and energy-efficient data communication.
Silicon-photonic devices are typically processed and fabricated on silicon-on-insulator (SOI) wafers. The device-fabrication process is usually conducted in wafer scale using conventional CMOS processes. In order to optically access the silicon-photonic devices, optical facets are formed at the end of the input and output optical waveguides. High quality optical facets are usually needed to achieve minimum optical coupling loss when coupling optical signals in and out of the silicon-photonic devices. Because an optical signal is prone to scattering if it is incident on a rough interface or a surface with contaminants, it is often important that the optical facets are smooth, contamination and defect free to ensure a low optical coupling loss. Extra optical coupling loss at the optical facets can significantly degrade the overall silicon-photonic device performance, and can make the silicon-photonic devices less efficient. Therefore, high-quality optical input/output facets are typically very important to achieving high performance silicon-photonic devices.
Usually, an optical facet is formed as the final step of the fabrication process after device patterning, etching, passivation and metallization. A variety of techniques is available for processing the optical facet. Traditionally, optical facets are processed by either end-polishing or cleaving. In the end-polishing process, the optical facet edge is roughly defined by dicing, and the diced area is then mechanically polished using a fine-grid polishing machine. This time-consuming approach is usually implemented at chip scale, and it typically does not define the optical facet position with high accuracy.
Alternatively, cleaving is a natural separation process along the crystalline orientation in the semiconductor substrate. Because of the pristine optical facet quality, cleaving is often the preferred technique to form optical facets on silicon-photonic devices. However, cleaving is a mechanical process with many restrictions. For example, cleaving can usually only be applied to the periphery of chips having a minimum device size. Furthermore, cleaving usually only allows coarse optical facet positioning, unless the substrate is very thin (less than 100 μm), which can make the substrate difficult to handle.
Another technique used to create optical facets is dry etching. Because the etching mask is patterned by photolithography, this approach allows accurate optical facet positioning compared to other mechanical techniques. Optical facets on many III-V compound semiconductor devices with reasonable performance are often fabricated using dry etching. However, the dry-etching technique typically requires careful selection of different etching chemistries for different material compositions. Furthermore, the etched profile and surface roughness are often very sensitive functions of the etching conditions. Consequently, fine adjustments to the dry-etching technique are typically needed for different chips. If an optical device includes multiple material layers, the dry-etching technique can be even more complicated because each material layer may need to be addressed individually. In addition, despite the successful use of the dry-etching technique for III-V compound semiconductor devices, an efficient optical facet fabrication technique for SOI-based silicon-photonic devices has not been developed.
Hence, what is needed is a technique for fabricating an optical facet without the problems described above.